Heretofore, in this type of electrostatic loudspeaker, the sound pressure can be produced corresponding to the driving signal by impressing the bias voltage (direct current biasing) with the driving signal.
More specifically, as shown in FIG. 1, in a loudspeaker system 1 utilizing an electrostatic speaker, the driving signal outputted from a power amplifier 2 is boosted several times via a boosting transformer 4 an output signal of which is outputted to a speaker element 6.
This speaker element 6 consists of stationary electrodes 8 and 10 which are electrode plates having multiple holes, or perforations, therein. These stationary electrodes are arranged opposing each other with a prescribed distance therebetween. The output signal of the boosting transformer 4 is impressed between these stationary electrodes 8 and 10.
Furthermore, the speaker element 6 includes a diaphragm 12 forming the conductive fine filter on the polyester film for example, and maintains this diaphragm 12 between the stationary electrodes 8 and 10.
The bias voltage of several (kV) is formed by boosting the commercial power source at a power source, between the secondary side middle tap of the boosting transformer 4 and the diaphragm 12.
The power source for forming bias voltage 14 impresses the bias voltage to the diaphragm 12 via resistance 16 having the prescribed resistance value, and thus, the loudspeaker system 1, as a whole, sets sharpness Q at the prescribed value.
With this arrangement, in the loudspeaker system 1, the driving force F to be expressed by the following equation is generated on the diaphragm 12; ##EQU1## and the sound pressure corresponding to the driving signal can be provided by elastically vibrating the diaphragm 12 with this driving force F.
Here, * is dielectric constant which, in this case, is 8.85 *10-12 [F/N], S is made up of electrode area (m2), EO and E are bias voltage [V] and voltage of driving signal [V] respectively, d0 is a distance between electrodes [m].
As shown in FIG. 2, the driving force F to be expressed by Equation (1) is produced on the entire diaphragm 12 equally.
On the other hand, in the diaphragm 12, since its member is homogeneous and also its outer circumference is restricted, it is deformed to a circular arc shape and is elastically vibrated as shown by the arrow "a" in FIG. 3.
This means that if the diaphragm 12 is vibrated with a large amplitude in order to obtain large sound pressure, the deformation of the diaphragm 12 becomes larger accordingly and this also means that the center part of the diaphragm 12 touches to the stationary electrodes 8 and 10.
In the speaker system 1, since the distance between stationary electrodes becomes larger, this contact can be avoided. However, if this distance d0 is made larger, it will become necessary to make the bias voltage and the voltage of the driving signal larger in accordance with Equation (1) and accordingly the efficiency becomes worse.
As shown in FIG. 4, this type of speaker system 19 in which the sound pressure would be increased by laminating multiple sets of speaker elements to be formed with a set of stationary electrode and diaphragm has been proposed as one method to obviate the above problems.
More specifically, the speaker system 19 includes the first speaker element with stationary electrodes 8A and 10A and the diaphragm 12A. The bias voltage is supplied to this speaker element through resistance 16A.
Furthermore, the speaker device 19 forms the second speaker element with stationary electrodes 8B and 10B and a diaphragm 12B and supplies the bias voltage to this speaker element via resistance 16B.
Thus, the speaker system 19 is able to produce larger sound pressure as compared with the case of forming with single speaker element.
However, in this speaker system 19, since the potential difference between stationary electrodes 10A and 8B of the first and the second speaker elements becomes large and also it is necessary to decrease the electrostatic capacity between these stationary electrodes 10A and 8B. After all, these stationary electrodes 10A and 8B have to be arranged estranged and as a result, it becomes a problem that its external form becomes large sized.
Furthermore, in the speaker system 19, if the stationary electrodes 10A and 8B are arranged estranged, the mass of air between stationary electrodes 10A and 8B would be increased accordingly; and as a result, the load of diaphragms 12A and 12B will increase.